<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button id='btn'>发送Ajax请求</button>
    <script>
        var btn = document.getElementById('btn');
        btn.onclick = function () {
            var xhr = new XMLHttpRequest();
            xhr.open('get', 'http://localhost:3000/error');
            xhr.send();
            xhr.onload = function () {
                console.log(xhr.responseText);
                if (xhr.status == 400) {
                    alert('请求出错')
                }
            }
            xhr.onerror = function () {
                console.log('断网了');
            }
        }
    </script>
</body>

</html>